Fuel cells are well known and are commonly used to produce electrical current from reducing fluid fuel and oxygen containing oxidant reactant streams, to power various types of electrical apparatus. Known solid oxide fuel cells (“SOFC”) generate both electricity and heat by electrochemically combining a fluid reducing fuel and an oxidant across an ion conducting electrolyte. In a typical SOFC, the electrolyte is an ion conductive ceramic membrane sandwiched between an oxygen electrode (cathode) and a fuel electrode (anode). Molecular oxygen, such as from the atmosphere, reacts with electrons at the cathode electrode to form oxygen ions, which are conducted through the ceramic membrane electrolyte to the anode electrode. The oxygen ions combine with a reducing fuel such as a mixture of hydrogen and carbon monoxide to form water and carbon dioxide while producing heat and releasing electrons to flow from the anode electrode through an electrical circuit to return to the cathode electrode.
Solid oxide fuel cells have many benefits and some limitations. For example, normal operating temperatures are very high, often in excess of 700° C., which favors stationary power plants operating in a near steady-state mode to minimize deleterious effects of thermal cycling as the fuel cell is started up and shut down. Efforts have been undertaken to increase the efficiency of such solid oxide fuel cells. For example, it is known to direct flow of unused fuel as an anode exhaust stream through an anode recycle loop, wherein unused fuel is directed from an anode exhaust line of the fuel cell, typically then through one or more heat exchangers, and back into an anode inlet upstream of the fuel cell. Additionally, all of the anode exhaust stream, or a portion of the anode exhaust stream returning from the anode recycle loop, or a mixture thereof is burned in a burner generating additional heat so that flammable exhaust does not pass out of the power plant.
While much of this heat may be productively utilized, unfortunately the extremely high temperatures reached by power plant components gives rise to many problems. First, when temperatures reach or exceed 900 degrees Celsius (° C.), exotic and very expensive materials, such as nickel-based super alloys must be utilized to address high oxidation rates, cracks during thermal cycling, high-temperature creep etc. It is beneficial to prohibit temperatures within the power plant from exceeding 800° C. This permits long-term usage of iron-based stainless steel, which is much less costly than nickel-based super alloys. If temperatures exceed 800° C., iron-based stainless steel alloys will suffer a chromium evaporation from a chromium oxide layer on the surface of the stainless steel, resulting in degradation of the SOFC stack. It is known that oxidation of the anode exhaust stream within the burner will raise a temperature of the burner above 900° C. This exceeds a maximum temperature of known inexpensive catalysts and catalyst support substrates commonly used in the automotive industry, therefore requiring very expensive materials within the burner and within heat exchangers that extract heat from exhaust gases leaving the burner, such as a cathode air heat-exchanger.
Many efforts have been undertaken to burn unused fuel in the anode exhaust streams of fuel cell power plants, while minimizing negative consequences of extremely high temperatures. U.S. Patent Application Publication US 2011/0053027 of Weingaertner et al. that was published on Mar. 3, 2011 shows at FIG. 2C use of a “ATO” apparatus to burn anode tail gas then direct the burned hot exhaust stream out of the apparatus through conduits to deliver heat to other power plant components. Additionally, U.S. Pat. No. 7,736,774 to Ogiwara et al. that issued on Jun. 15, 2010 shows at FIGS. 11-18 various embodiments of a heat exchanger 26 that includes a catalyzed burner 25 within the heat exchanger. The burner directs the hot exhaust stream out of the burner and back into an adjacent cathode air or anode fuel heat exchanger. Another effort to utilize extremely hot temperatures of a combusted anode exhaust stream that is shown in U.S. Pat. No. 6,920,920 that issued on Jul. 26, 2005 to Whittenberger. This patent shows a heat-exchanger having oxidation catalysts located at various positions along one side of a metal foil to ignite combustible gases in a heat exchanger utilized in operating a catalytic fuel reformer.
While such efforts show varying usage of extremely hot, combusted anode gas streams, all of these and other known efforts result in a requirements for usage of very costly, high-temperature tolerant materials, especially when used in association with SOFC power plants that are most efficient at very high operating temperatures.